Encapsulated breaker chemical with a multi-coat layer urea

ABSTRACT

An encapsulated breaker chemical composition used in a fracturing process in an oil or gas stimulation operation which comprises: (a) a breaker chemical; (b) a water soluble sealing layer, such as urea, having a thickness within the range of about 1 to about 30 microns deposited on the surface of said breaker chemical and encapsulating said breaker chemical; and (c) a pinhole free coating of a neutralized sulfonated elastomeric polymer of about 2 to about 80 microns thick deposited onto the surface of the coated breaker chemical, wherein said neutralized sulfonated polymer encapsulates the breaker chemical and is permeable to the breaker chemical.

This application is a continuation-in-part of copending applicationsSer. Nos. 07/446,572 and 07/446,573, filed on Dec. 4, 1989, both nowabandoned, and Ser. No. 07/446,736, filed on Dec. 6, 1989, nowabandoned.

FIELD OF THE INVENTION

The present invention relates to encapsulated breaker chemicals whichare used in fracturing fluid systems. The encapsulated breaker chemicalcomprises a breaker chemical which is encapsulated within a sealinglayer which is chemically inert with respect to said breaker chemical,which structure is further encapsulated within an elastomeric polymericcoating which is suitably permeable to permit the breaker chemical to bereleased through the polymeric coating into the fracturing fluid. Thepolymeric coating of the instant invention comprises a neutralizedsulfonated elastomeric polymer.

DESCRIPTION OF PRIOR ART

Solids (e.g., substrates, pipes, slabs, sheets, etc.) can be protectedfrom the external environment with the use of barrier or protectivecoating materials. For protection from water or moisture, polymer ororganic materials are widely used. For coat effectiveness, however,these materials are generally applied as thin films. The thickness ofthe film depends upon the desired degree of water protection. Thethicker the film the more likely that water penetration would be sloweddown. In practice, applying an effective thin coating is difficultbecause of the various stresses tending to make the film discontinuous(e.g., film-rupture, pin holes). Films will rupture when a thresholdstress is exceeded. The lateral stress tending to rupture a film isinversely proportional to an exponential power of the film thickness.The thinner the film, the more easily it will rupture.

There are many applications for thickened or gelled solutions ofpolymers in aqueous or organic liquids which are quite diverse. Thereare also a number of physical and chemical techniques for preparing suchsystems. Such systems are used for viscosity control, solids carryingand fluid loss control in fracturing fluids or to place sand or gravelpacks downhole in oil or gas wells.

Many types of fluids are available for rise in wellbore fracturingprocedures. Chief properties required by the fracturing fluids are lowleak off rate, the ability to carry a propping agent and low pumpingfriction loss. The fluid should also be easy to remove from theformation; it should be compatible with natural formation fluids; and itshould cause a minimum of damage to the formation permeability. Theseproperties can be achieved with oil based fluids, water based fluids,emulsions, foam, and other multi-phase suspensions.

A fracturing process used in a well treating operation involves theintroduction into the well of a fluid under sufficient hydraulicpressure to produce or enlarge fractures in the subterranean formation.The fracturing fluid usually contains a propping agent such as sand,sintered alumina, glass beads, aluminum spacers, urea prills ornutshells, wherein the propping agent is deposited within the fracturein order to maintain the fracture in an open mode or alternatively, anacid is contained within the fracturing fluid wherein the acid reactswith acid reactable components of the subterranean formation therebycreating and enlarging the fracture so that one obtains increased flowof fluids from the subterranean formation. The propping agent issuspended within a polymeric gel or a highly viscous polymeric solution.After the highly viscous fluid has been pumped into the fracture and thefracture is formed or enlarged, it is desirable that the highly viscousfluid be converted into a low viscosity fluid so that the fluid can beremoved from the fracture thereby leaving the propping agent within thefracture. The removal of the fluid is necessary so that the oil or gascan flow back through the fracture into the well bore. The conversion ofthe highly viscous solution into the lower viscosity fluid is calledbreaking the gel. The breaking of the gel is accomplished by adding abreaker chemical to the gel at the appropriate time. It is critical thatthe breaking process does not prematurely occur prior to the completionof the fracturing process. The usual method of addition of the breakerchemical is to pump it through the well bore into the fracture uponcompletion of the fracturing process, but this presents the problem ofhaving the breaker chemical completely mix with the viscous fluid withinthe fracture to maximize breaking of the gel.

The use of breaker systems to break viscous water gels and fluids is notof itself new. Systems describing these chemicals are found, forexample, in U.S. Pat. Nos. 3,167,510; 3,816,151; 3,960,736; 4,250,044and 4,506,734. But most of these breakers are applied after introductionof the fracturing fluid into the wellbore because these breakersimmediately start to degrade the viscosity enhancer in the fracturingfluid upon contact thereby lowering the efficiency of the fracturingprocess. In such cases, additional time and labor are needed to effectthe reduction of the viscosity of fracturing fluids introduced into thesubterranean formation. The use of organic breakers such as alkylformate may alleviate this problem, since they can be applied along withthe fracturing fluid. But these types of breakers rely on certainsubterranean conditions, such as elevated temperature and time, toeffect a viscosity reduction of the fracturing fluid. Since theseorganic breaker chemicals work on chemical change, such as hydrolysis,they are slow in effecting viscosity reduction. Furthermore, theirperformance can be unpredictable.

Accordingly, the incorporation of a breaker chemical into the fracturingfluid prior to the pumping of the fracturing fluid into the well boreand well fractures is desirable. The breaker chemical must be in apassive, non-reactive state such that it cannot react with the viscousfluid of the fracturing well prior to the injection of the fracturingfluid into the fractures, but the breaker chemical must be capable ofreacting with the viscous fluid of the fracturing fluid within thefracture upon the completion of the fracturing process. The presentinvention teaches a method of accomplishing this objective.

SUMMARY OF THE INVENTION

The present invention relates to encapsulated breaker chemicals whichare used in a process for forming and enlarging fractures in asubterranean formation in oil well stimulation operations. Theencapsulated breaker chemical is added above ground to the fracturingfluid which comprises a viscous or gelled polymeric solution, a proppingagent, the encapsulated breaker chemical and other additives commonlyused in fracturing fluids. The encapsulated breaker chemical comprises abreaker chemical having a sealing layer deposited on and encapsulatingthe breaker chemical, and an elastomeric coating deposited on thesurface of the sealing layer, wherein the elastomeric coating, having athickness of at least about 1 micron, preferably from about 1 to about100 microns, more preferably from about 2 to about 80 microns, and mostpreferably from about 5 to about 70 microns encapsulates the sealinglayer, and the sealing layer having a preferred thickness of from about1 to about 30 microns encapsulates the breaker chemical. The elastomercoating must be made from a material which will be permeable to thebreaker chemical under conditions of use. The coating of the instantinvention comprises a neutralized and ionically cross linked sulfonatedelastomeric polymer, preferably a neutralized sulfonated terpolymer ofethylene, propylene and a non-conjugated diene (EPDM terpolymer).

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a plot of release versus immersion time in water forthe coatings of the instant invention and control samples.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an encapsulated breaker chemicalcomposition used in a fracturing process in the production of oil or gaswhich comprises: (a) a breaker chemical; (b) a sealing layer or coatingencapsulating the breaker chemical; and (c) a pinhole free coating of aneutralized sulfonated elastomeric polymer having a preferred thicknessof about 2 to about 80 microns deposited onto the surface of the sealinglayer wherein said neutralized sulfonated polymer encapsulates saidsealed breaker chemical, wherein said neutralized sulfonated polymerwill be permeable to said breaker chemical under conditions of use andsaid neutralized sulfonated polymer is substantially non-reactive tosaid breaker chemical.

The sulfonated (sulfo) elastomeric polymers which are preferred for thepurposes of this invention are water insoluble and include sulfonatedcopolymers of ethylene and propylene, sulfonated terpolymers ofethylene, propylene and a diene, preferably a non-conjugated diene(EPDM), sulfo Butyl rubber, sulfo isoprene/styrene rubber, sulfoisoprene/butadiene rubber, sulfo isoprene/butadiene/styrene copolymers,sulfo isobutylene/styrene copolymers, sulfo isobutylene/para methylstyrene copolymers, and complexes of the aforementioned polymers with anitrogen containing polymer such as vinyl pyridine copolymer.

The more preferred neutralized sulfonated elastomers of the instantinvention are selected form the group consisting of sulfonated Butylrubber, sulfonated EPDM terpolymer and a complex of sulfonated (PDMterpolymer with a vinylpyridine copolymer.

The water insoluble sulfonated polymers of the instant invention willcomprise from about 4 to about 200 milliequivalents of pendant sulfonategroups per 100 grams of polymer, more preferably from 10 to 100 meq.pendant sulfonate groups. The sulfonated elastomers utilized in theinstant invention are neutralized with transition elements selected fromGroups IVA, VA, VIA, VIIA, VIIIA, IB, and IIB of the Periodic Table ofElements and lead, tin, and antimony, as well as ammonium and aminecounterions. Zinc and sodium neutralizing ions (as ionic salts, e.g.,acetate or hydroxide) are preferred.

Neutralization of the cited polymers with appropriate metal hydroxides,metal acetates, metal oxides or ammonium hydroxide, etc. can beconducted by means well known in the art. For example, the sulfonationprocess as with Butyl rubber, containing a small 0.3 to 1.0 mole percentunsaturation, can be conducted in a suitable solvent, such as toluenewith acetyl sulfate as the sulfonating agent such as described in U.S.Pat. No. 3,836,511, incorporated herein by reference. The resultingsulfonic acid derivative can then be neutralized with a number ofdifferent neutralization agents such as sodium phenolate and similarmetal salts. The amounts of such neutralization agents employed willnormally be stoichiometrically related to the amount of free acid in thepolymer. It is preferred that the amount of neutralizing agent employedis in an excess percent to ensure full neutralization. The use of anexcess of such neutralization agent is necessary to effect at least 50percent neutralization of the sulfonic acid groups present in thepolymer, preferably at least 90 percent, and most preferably essentiallycomplete neutralization of such acid groups should be effected.

Thus, the degree of neutralization of said sulfonate groups may varyfrom 50 to 100 mole percent. With the utilization of neutralizedsulfonates in the instant invention, it is preferred that the degree ofneutralization be substantially complete, that is, with no substantialfree acid present and without substantial excess of the base, other thanthat needed to ensure neutralization. The neutralized sulfonates possessgreater thermal stability compared to their acid form and are ionicallycrosslinked. Thus it is clear that the polymers which are normallyutilized in the instant invention comprise substantially neutralizedpendant groups and, in fact, an excess of the neutralizing material maybe utilized without defeating the objects of the instant invention.

The sulfonated elastomer polymers of the instant invention may vary innumber average molecular weight from 1,000 to 10,000,000 preferably5,000 to 1,000,000 most preferably from 10,000 to 600,000. Thesepolymers may be prepared by methods known in the art, for example, seeU.S. Pat. No. 3,642,728, incorporated herein by reference.

The most preferred sulfonated polymers for use in the instant inventionare sulfonated ethylene/propylene terpolymers which may be prepared bythe procedures described in U.S. Pat. No. 3,870,841, incorporated hereinby reference.

The sulfonated polymers of the instant invention may be neutralizedprior to incorporation into organic solvent or by neutralization of theacid form in situ. Preferably the acid derivative is neutralizedimmediately after preparation. For example, if the sulfonation of anEPDM terpolymer is conducted in solution, then the neutralization ofthat acid derivative can be conducted immediately following thesulfonation procedure. The neutralized polymer may then be isolated bymeans well known to those skilled in the art, i.e., coagulation, steamstripping, or solvent evaporation, because the neutralized polymer hassufficient thermal stability to be dried for employment at a later timein the process of the instant invention. It is well known that someunneutralized sulfonic acid derivatives do not possess good thermalstability and the above operations avoid that problem.

The concentration of the neutralized sulfonated polymer in the solventsolution is about 0.1 to about 20 weight percent, more preferably about0.5 to about 6.0 weight percent. As discussed above, a variety ofpolymer backbones will display the desirable properties discovered inthis invention.

Specific examples of organic liquids to be employed as solvents for thepolymers include:

    ______________________________________                                        Polymer           Organic Liquid                                              ______________________________________                                        Sulfonated ethylene-                                                                            aliphatic and aromatic                                      propylene terpolymer                                                                            solvent oils such as                                                          Solvent "100 Neutral",                                                        "150 Neutral", and                                                            similar oils; benzene;                                                        diesel oil; toluene;                                                          xylene; ethyl benzene;                                                        pentane; hexane;                                                              heptane; octane;                                                              isooctane; nonane;                                                            decane; and ketone                                                            solvents.                                                   sulfonated polystyrene                                                                          benzene, toluene, ethyl                                                       benzene, methyl ethyl                                                         ketone, xylene,                                                               styrene, ethylene                                                             dichloride, and                                                               methylene chloride                                          ______________________________________                                    

In order to reduce the viscosity of the organic hydrocarbon solution ofthe sulfonated polymer so as to be able to employ the organichydrocarbon solution in a fluidized bed process, a polar cosolvent maybe added to the organic hydrocarbon solution of the sulfonatedelastomeric polymer to solubilize the pendant ionomeric groups. Thepolar cosolvent will have a solubility parameter of at least 10.0, morepreferably at least 11.0, and may comprise from 0.0001 to 15.0 weightpercent, preferably 0.001 to 5.0 weight percent, of the total mixture oforganic liquid, water insoluble ionomeric polymer and polar cosolvent.

Normally, the polar cosolvent will be a liquid at room temperature,however, this is not a requirement. It is preferred, but not required,that the polar cosolvent be soluble or miscible with the organic liquidat the levels employed in this invention. The polar cosolvent may beselected from the group consisting of alcohols, amines, di- ortri-functional alcohols, amides, acetamides, phosphates, or lactones andmixtures thereof. Especially preferred polar cosolvents are aliphaticalcohols such as methanol, ethanol, n-propanol, isopropanol, 1,2-propanediol, monoethyl ether of ethylene glycol and n-ethylformamide.

The coating of any solid particulates such as the breaker chemical offracturing fluid breaker systems can be achieved by spraying in afluidized bed process a solution of sealing layer (such as a aqueoussolution of urea) onto a cascading stream of granules to be coated, anddrying said granules, to achieve a sealing layer thickness within therange of from about 1 to about 30 microns. The coated granules may thenbe subsequently overcoated with the neutralized elastomer, such as azinc sulfo EPDM terpolymer, using a solution of the elastomer inappropriate solvent. Examples of fluidized bed coating processes areconventional spray coating wherein the solid particulates are coated byspraying the coating solution above or below the bed in a Wursterconfiguration or a fluidized bed with a rotating bed support plate. Itis envisioned that coated solid inorganic or organic breaker systems orother particulates can be utilized in a variety of environmentalconditions and yet the release of the breaker or other water solublecomponent of the breaker can be controlled in such a way that they areavailable when the reduction of viscosity is needed.

The neutralized sulfonated elastomers of this invention are slowlypermeable to water and essentially impermeable to the breaker chemicalsmentioned herein under well-bore conditions. Upon introduction intoaqueous fracturing fluids or other aqueous wellbore fluids, theencapsulated substrate slowly absorbs water by diffusion through thepolymeric coating. This water dissolves the substrate and sets up anosmotic gradient which in turn draws in more water. Pressure builds upinside the capsule, and it expands until resealable micropores (notpinholes) form in its walls. Concentrated substrate solution is thenejected through the micropores into the surrounding medium. This relivesthe pressure inside the capsule which then shrinks. The microporesreseal, and the process repeats itself until insufficient substrateremains for swelling and micropores to form.

Specific examples of preferred breaker chemicals of the instantinvention are selected from the group consisting of ammonium and alkalipersulfates, alkyl formates, salicylates, acetates, chlorites,phosphates, laurates, lactates, chloroacetates, enzymes and other solidbreakers. The release of the breakers from the coated solid breakersystems can be controlled by the thickness of the sulfonated polymercoating, the degree of sulfonation of the polymer, the degree ofneutralization of the sulfonated groups of the sulfonated polymers, thetype of cation in the sulfonated polymer, the biodegradability of thesulfonated polymers, and the intrepidity and conformity of applicationof the polymers.

The thickness of the coating of the neutralized sulfonated elastomericpolymer on the coated breaker chemical is about 1 to about 100 microns,more preferably about 2 to about 80 microns, and most preferably about 5to about 70 microns.

The chemical forming the encapsulated breaker chemical may be used perse when it is in the form of a finely divided solid or, in anotherembodiment of the invention, it may be sprayed as a solution or inliquid form onto small, finely divided seed particles to form a coatingon these seed particles. Essentially any solid which is of the propersize and which is inert to the breaker chemical (or other activematerial) may be used as the seed particle, but urea is preferred. Thisembodiment is especially preferred where the breaker chemical is itselfa liquid, or is irregular in shape or not of the proper size.

In another embodiment of this invention, the encapsulated breakerchemical, with or without a seed core, may be further overcoated byprocesses described above with a solution of thermoplastic polymermaterial which exhibits permeability properties similar to those of theneutralized elastomer coating overlying the breaker chemical. Thisovercoat layer is particularly useful in reducing the tackiness of theelastomer layer and thereby allows for the application of a thickerelastomer layer. For example, a sulfonated EPDM elastomer layer maybecome tacky during the coating process and the coated particles tend tostick together. The application of an overcoat layer of thermoplasticpolymer removes the tackiness of the previously coated particles so thatthicker layers of the sulfo EPDM coating may be achieved.

Preferred thermoplastic overcoat layers are based on sulfonated styrenepolymers or copolymers which are also neutralized to the degree setforth above with respect to the elastomeric polymers. The thickness ofthe thermoplastic layer is generally less than the thickness of theelastomeric layer, and may range from about 1 to about 20 microns, morepreferably from about 1 to about 10 microns, and most preferably fromabout 1 to about 5 microns.

Thermoplastic polymers which are most preferred as overcoat layers inaccordance with this invention include neutralized sulfonatedpoly-styrene and neutralized sulfonated poly (p-methyl) styrene having adegree of acid neutralization and a molecular weight within the rangesspecified above for the elastomeric polymer.

Coating of the neutralized sulfonated elastomer, such as an EPDMterpolymer and the thermoplastic polymer, such as a neutralizedsulfonated polystyrene polymer, on breaker chemicals coated with asealing layer protects the breaker from aging and degrading the polymercoating. These coatings also function in the fracturing fluids as abarrier to water soluble components of the fracturing fluid therebyshielding the breaker chemical from the premature release of the breakerchemical into the fracturing fluid.

The protective layer is most important when an oxidizer is the breakersince it protects the breaker chemical from degrading the polymericcoating. It is contemplated that any solid that is inert to the breakerand the neutralized sulfonated EPDM terpolymer may be used as theprotective layer as long as it is water soluble and does not change thebreaker release through the neutralized sulfonated EPDM terpolymer andthe neutralized sulfonated polystyrene polymer. Urea is most preferredas the protective layer.

The preferred breaker chemicals which may be encapsulated includeammonium persulfate, sodium persulfate, potassium persulfate, sodiumchlorite, ammonium bifluoride, ammonium fluoride, sodium fluoride,potassium fluoride, sulfamic acid, citric acid, oxalic acid, ammoniumsulfate, sodium acetate and enzymes and mixtures thereof.

The breaker chemical can also be used in combination with a polymericmatrix as described in U.S. Pat. No. 4,738,897 incorporated herein byreference, absorbed on a porous solid such as diatomaceous earth orcoated on an inert core substrate such as urea as described above.

Other oil well chemical additives can be encapsulated in addition to orin place of the breaker chemicals. These materials when encapsulatedwill be administered under the same conditions and in the same ways asin normal use. These oil well chemical additives are selected from thegroup consisting of: scale inhibitors such as polyacrylates,phosphonates and phosphate esters; corrosion inhibitor such as acidamine salts, imidazolines and quaternary amines; demulsifiers such asalkoxylated resins, alkoxylated polyols and alkoxylated polyesters;biocides such as glutaraldehydes, isothiazolinones and dithiocarbamates;paraffin inhibitors such as ethylene vinyl acetate copolymers, alphaolefin maleate and furmarate polyesters, and vinyl acetate; foamers suchas ether sulfates, alpha olefin sulfonates and quaternary amines; cementaccelerators such as calcium chloride and aluminum chloride; and gasmigration agents such as latex rubbers and aluminum powders.

It is also within the scope of this invention that other inorganic andorganic materials may be encapsulated with the polymeric coating systemof this invention such as materials used in agricultural applications,wherein these agricultural materials are selected from the groupconsisting of urea fertilizers, herbicides, pesticides and mixturesthereof.

In order for the elastomeric coatings to be functional, they have tomeet one or more of the following criteria: the coating material shouldshow improved barrier properties; the applied thin coating should be acontinuous film with few or no defects, i.e., be substantially free ofpinholes. The term "pinholes" as used herein is understood to meandiscontinuities in the polymeric coating of such size and shape as toprevent osmotic pressure buildup.

Fracture fluids commonly used in fracturing operations includewater-based brine fluids containing a water-soluble polymer such ashydroxypropyl guar cross-linked with a transition metal salt as is wellknown in the art. Other polymers used to viscosify aqueous fracturingfluids are hydroxyethyl cellulose, polyacrylamide, gum karaya and thelike. In addition, fracturing fluids may be viscous oils or oil-basedemulsions. Viscosification of these is accomplished via addition ofsurfactants, asphalts, fatty-acid soaps, other emulsion and foamstabilizing agents and the like.

Typical propping agents used in hydraulic fracturing for retaining theintegrity of the fractures are nut shells, plastic beads, glass beads,sand, sintered alumina, urea prills and aluminum spacers. Theconcentration used depends on the size and shape of the proppant, thetype of formation being fractured, the carrying capacity of thefracturing fluids and the desired permeability of the resistantfractures. The concentrations generally range from 1 to 30 lb. per gal.

Other additives commonly used in hydraulic fracturing fluids are fluidloss or wall building agents such as starch, bentonite, silica flour,guar gum and surfactants; friction-reducing agents such as small amountsof high molecular weight linear polymers such as polyacrylamide;specific gravity increasing agents; bactericides;scale-removal/prevention agents, and surfactants or alcohol to reduceinterfacial tension and the resistance to return flow.

The following Examples are illustrative of the invention.

EXAMPLE 1

This example sets forth the preparation of two sulfonated EPDMterpolymers which are subsequently neutralized to form the zinc salt(SEPDM).

Sulfonated EPDM terpolymers are prepared by sulfonating a terpolymercomprising about 50 weight percent ethylene, about 5 weight percentethylidene norbornene (ENB) and the balance being propylene. Thismaterial is available from Uniroyal under the trademark "ROYALENE" 521.Sulfonation was carried out by treatment with acetyl sulfate andneutralization with ZN (Ac)₂ according to the procedure of U.S. Pat. No.3,836,511, herein incorporated by reference.

Two species of neutralized elastomer were produced, one having 10 meq.of zinc sulfonate groups per 100 grams of the EPDM terpolymer(hereinafter designated ZSE-10), and the other having 25 meq. of zincsulfonate groups per 100 grams of the (PDM terpolymer (hereinafterdesignated ZSE-25).

EXAMPLE 2 Improved Barrier Properties of a Sulfonated Polymer

A solution of a zinc neutralized sulfonated EPDM polymer was preparedfor application as a coating solution. The solution cast coating wasapplied over urea samples in order to measure the barrier properties forurea extraction by water through the thin film coating.

Solution A was prepared by mixing 2.5 grams of a zinc sulfo-EPDM similarto the one described in Example 1 (Uniroyal Ionic Elastomer 2560),containing 25 meq. of sulfonate groups per 100 grams of ethylenepropylene ethylidenenorbornene terpolymer, neutralized with zinc, in 100grams of a solvent consisting of 97.5% toluene and 2.5% methanolreagent.

To determine the barrier properties of films formed from the solution ofthe sulfonated EPDM terpolymer, urea slides were coated for immersiontests. The procedures for preparing coated samples of urea slides andconduction immersion tests are described as follows:

Urea samples were prepared by depositing reagent grade urea (FischerScientific) over microscope glass slides. This was done by dipping glassslides into molten urea at a temperature of about 135°-145° C., followedby cooling and solidification of the urea layer. The urea layer wasbuilt up to abort 7 mm. by 4 to 5 successive dipping and cooling cycles.These urea samples were then coated by a polymeric film using a seconddipping procedure. Urea slides were repeatedly dipped into polymericsolutions such as those described above, followed by drying in a vacuumoven at 70° C. for about 3 hours. The dipping and drying cycles wererepeated until the film thicknesses shown in Table I were obtained.

The barrier properties of the various polymeric films were determined byimmersion of each coated urea slide in about 100 g. of deionized waterat room temperature. The amount of urea released into the water wasdetermined by recovering the urea after evaporating the water. Eachsample was initially immersed for 1 day, followed by immersion in freshwater for 3 days and for weekly intervals thereafter.

Table I shows the permeabilities of urea solutions extracted from thecoated urea slides and immersed in water at room temperature. Thepermeabilities of the coating materials were determined by applyingFick's law of diffusion at steady state. Fick's law states that:##EQU1## where J_(m) =mass flux (loss) through the film or membrane,A=transport area, C concentration gradient, X=film or membranethickness, and D=membrane diffusivity constant which is equal to theratio of permeability (P) over the solubility ratio (K) of urea in themembrane and in water.

The performance of the zinc-sulfo-EPDM film was compared with thatformed from solutions of other commercial coating materials. The firstcommercial coating solution was a tung oil solution made by

                  TABLE I                                                         ______________________________________                                        PERMEABILITY OF UREA SOLUTION FROM COATED                                     UREA SLIDES IN WATER AT ROOM TEMPERATURE                                                                          Permeability                              Sample                                                                              Coating    Ratio    Film Thickness                                                                          (P = DK)                                  No.   Material   wt/wt    Microns   cm.sup.2 /sec                             ______________________________________                                        141-3 Tung Oil   --       75        4.3 × 10.sup.-9                     141-6 Tung Oil   --       125       7.6 × 10.sup.-9                     158-4 Polyurethane                                                                             --       100       1.3 × 10.sup.-9                     158-5 Polyurethane                                                                             --       40        2.1 × 10.sup.-9                      28-14                                                                              ZSE-25     --       10        9.6 × 10.sup.-13                     84-1 ZSE-25     --       25        8.6 × 10.sup.-13                     84-2 ZSE-25     --       30        8.6 × 10.sup.-13                    ______________________________________                                    

Formby of Mississippi at 30 weight percent solids in petroleumdistillate. The second commercial coating solution was linseed oilmodified polyurethane Type I made by Minwax Paint Co. of New Jersey at45% solids in petroleum distillate. The two commercial coatings werecured at 70° C. for 48 hours after coating.

The permeability of urea solutions through the zinc sulfonated EPDMterpolymer film was found to be about 3 orders of magnitude lower thaneither that of tung oil or that of polyurethane. The use of tung oil andpolyurethane as release control coatings on water soluble fertilizers isdisclosed in U.S. Pat. Nos. 3,321,298 and 3,223,518.

The present Example shows that thin films of zinc sulfonated EPDMterpolymer complex provide a much better barrier than do commercialmaterials.

EXAMPLE 3

In this Example, thick coatings of reagent sulfur and paraffin wax(M.P.=55° C.) were applied through melting (i.e., these materials weremelted at about 5° C. to 10° C. above their melting point and then afterquickly dipping urea samples in the melt, the hot coated sample wasbrought to room temperature slowly). This layering procedure wasrepeated three to four times to obtain a thick coating. Bothunsulfonated EPDM and PEVAC (a copolymer of ethylene and vinyl acetatecontaining about 60 weight percent ethylene) were applied from 2.5%polymer solutions in the same solvent as the solvent for the sulfo EPDM.

FIG. 1 shows that ZSE-10 offered the best protection preventing thedissolution of urea in water at room temperature (about 22° C.).Uncoated urea dissolved quickly in water, but even thick coatings ofsulfur, unsulfonated EPDM and PEVAC on similar samples did notsufficiently protect urea from dissolving in water. Paraffin wax andmodified polyurethane Type 1 (made by Minwax Paint Company) protectedurea from dissolving in water for at least six days. ZSE-10, however,performed the best in protecting urea from water dissolution.

EXAMPLE 4 Fluidized Bed Process for Urea Coating

The urea coated sodium persulfate granules were produced using thefollowing procedure. 4 kg of 20-30 mesh sodium persulfate wereintroduced into a fluid bed coating machine, including a Wurster insert,manufactured by Glatt Air Techniques Inc., model number GPCG-5. Thesodium persulfate bed was fluidized by passing 130 scfm heated air (48°C.) through the bed. After the bed reached a temperature of 30° C., a 10weight percent solution of urea in water and methanol was sprayed intothe sodium persulfate granules at the Wurster insert entrance. The spraynozzle used was a commercial two fluid nozzle using air at 3 barspressure to form an atomized spray regime in the Wurster insert.

The spraying was continued at 20 g/min rate until the required thicknessof urea was built up on the sodium persulfate, i.e. approximately 46minutes for a coating level of 2 wt. % urea on the sodium persulfate.

After the solution was sprayed onto the granules in the Wurster insert,the thinly coated granules were blown by the heated air upwards into thedrying section of the machine. Here, the solvents were evaporated by thehot stream, leaving a thin coat of urea on the granules. The driedgranules fell back into the fluid bed and then reentered the Wursterinsert where the coating process was repeated. Thus, multiple films andlayers of the urea were built up until the spraying was stopped at a 2wt % coating.

EXAMPLE 5 Fluidized Bed Process for SEPDM Coating

Urea coated sodium persulfate granules coated with SEPDM were producedusing the following procedure:

4 kg of 20-30 mesh urea coated sodium persulfate granules wereintroduced into a fluid bed coating machine, including a Wurster insert,manufactured by Glatt Air Techniques Inc., model number GPCG-5. Thegranular bed was fluidized by blowing 130 scfm of heated air (45° C.)through the bed. After the bed reached a temperature of 30° C., a 1.25weight percent solution of the SEPDM polymer in toluene and methanolcosolvent was sprayed onto the granules at the Wurster insert entrance.The spray nozzle used was a commercial two fluid nozzle using air at 3bars pressure to form an atomized spray regime in the Wurster insert.

The spraying was continued at 40 gm/min rate until the requiredthickness of polymeric coating was built up on the granules, i.e.,approximately 80 minutes per a coating level of 1 weight percent polymeron the urea coated sodium persulfate.

After the solution was sprayed onto the granules in the Wurster insert,the thus coated granules were blown by the heated air upwards into thedrying section of the machine. Here, the solvents were evaporated by thehot stream, leaving a thin coat of dried polymeric material on thegranulas. The dried granules fell back into the fluid bed and thenreentered the Wurster insert where the coating process was repeated.Thus, multiple films or layers of the sulfonated polymeric coating wasbuilt up until the spraying was stopped.

After a 1 weight % coating was put on the granules, a 60 gram sample wastaken and marked as 1 wt % of SEPDM on urea coated sodium persulfate.The spraying was continued until 2wt % SEPDM was added. The spraying wasthen stopped and the coated granules were dried with the hot air for 5minutes.

EXAMPLE 6

2.34 grams of 20 to 30 mesh urea coated granulated sodium persulfatecoated with 1 and 2 weight percent SEPDM terpolymer as set forth inExample 5, were placed in 30 grams of a 30 weight percent bariumchloride solution. The sodium persulfate converts to approximately 5.5grams of barium sulfate, as shown by the uncoated persulfate data at250° F./121° C. The coated solids, after remaining at the desiredrelease temperature and time, were separated from the finer bariumsulfate solids and the water by pouring through a 40 mesh sieve. Thesieve was thoroughly rinsed with water to remove any solids or liquidsclinging to coated solids. The barium sulfate in water plus any watersoluble salts were left overnight to separate. The free water was thendecanted from the solid barium sulfate. The remaining water was removedby drying in an oven at 150° F. (65° C.) overnight. The remaining bariumsulfate was weighed and recorded in Table II. The percent sodiumpersulfate released is given by using the uncoated weight at 100%released.

                                      TABLE II                                    __________________________________________________________________________    Release of Coated Sodium Persulfate in Water at Time and Temperature                            Release of Coated Sodium Persulfate by Weight in grams                        of                                                                            Converted Barium Sulfate and by Percent versus Time in                        hours                                                       Coated Sodium                                                                            Temperature                                                                          4         8         16                                      Persulfate (°F./°C.)                                                              weight (g)                                                                          %   weight (g)                                                                          %   weight (g)                                                                          %                                 __________________________________________________________________________    Sodium Persulfate                                                                        75/24  0.83  100 0.88  100 0.86  100                               with 2 wt % urea                                                                         150/65 4.79  100 4.22  100 4.95  100                                          250/121                                                                              5.30  100 5.34  100 5.20  100                               Sodium Persulfate                                                                        75/24  0.28  34  0.61  69  0.78  91                                with 2 wt % urea                                                                         150/65 1.36  28  2.91  69  4.42  89                                Plus 1 weight                                                                            250/121                                                                              4.40  83  4.38  82  4.27  82                                Percent S-EPDM                                                                Coating                                                                       Sodium Persulfate                                                                        75/24  0.25  30  0.48  55  0.59  69                                with 2 wt % urea                                                                         150/65 1.01  21  2.64  63  4.26  86                                Plus 2 weight                                                                            250/121                                                                              4.35  82  4.23  79  4.28  82                                Percent S-EPDM                                                                Coating                                                                       Sodium Persulfate                                                                        75/24  0.25  30  0.42  48  0.64  74                                with 2 wt % urea                                                                         150/65 1.13  24  2.30  55  6.20  85                                plus 2 wt % SEPDM                                                                        250/121                                                                              4.40  83  4.35  81  6.45  86                                plus 1/2 wt % SPS                                                             __________________________________________________________________________

EXAMPLE 7

The following Example illustrates the preparation of a sodium sulfonatedpolystyrene thermoplastic polymer (SPS) which may be used as anadditional layer over the SEPDM layer.

Sulfonated polystyrene is manufactured as a 30% suspension/emulsion inwater. The monomers of sodium styrene sulfonate (NaSS) and styrene areadded to water (oxygen free) containing surfactant (sodium laurylsulfate), catalysts (potassium persulfate and sodium bisulfate) andchelates (EDTA) for iron. The mixture is reacted at 180° F. (82° C.) for3 hours. After the reaction is complete, an antioxidant (BHT) is added.The final product is 30% SPS suspended/emulsified into water. Thepolymer is oil soluble since only 3.8% NaSS is present in the polymer.

EXAMPLE 8

The multilayer coating with sulfonated polystyrene polymer is achievedby spraying the sodium sulfonated polystyrene (SPS), as prepared inExample 7 or a dilution thereof, onto the cascading stream of granulescoated as described above with the sulfonated EPDM terpolymer. The SPSpolymer is a suspension/emulsion in water so it can easily be dilutedwith water to the proper level for spraying multilayer coats.

After 2 wt. % of SEPDM was put on the sodium persulfate as set forth inExample 6, a 1/2 wt. % of SPS was added as a top layer. The conditionswere kept the same as in Example 5 above except that a 5 wt. % SPSsolution, a dilution of 30 wt. % SPS solution in water, was sprayed ontothe 2 wt. % SEPDM coated granules at a rate of 40 gm/min. Afterapproximately 9 minutes, a 1/2% SPS coating was completed. The sprayingwas stopped and the product was dried with hot air another 5 minutes.The product comprised 2 wt. % SEPDM plus 1/2 wt. % SPS on sodiumpersulfate.

The release properties of this material were also evaluated as inExample 5 above and are reported as the last entry in Table II.

EXAMPLE 9

Sodium persulfate was layered/coated on urea seed granules by using thefollowing procedure: 1.6 kg of 2.5 mm OD urea granules were introducedinto a fluid bed coating machine with a 2 liter bowl, manufactured byGlatt Air Techniques Inc., model number GPCG-1. The urea bed wasfluidized by blowing approximately 130 scfm of heated air (45° C.)through the bed. After the bed reached 30° C., a 10 wt % sodiumpersulfate solution in water and methanol was sprayed onto the ureafluidized to the top of the bowl with a top spray nozzle. The spraynozzle used was a commercial two-fluid nozzle using air at 2 barspressure to form an atomized spray regime in the bowl.

The spraying was done at 20 gm/min and continued until approximately a 5wt % coating of sodium persulfate was added. The spraying tookapproximately 40 minutes to add the 5 wt % level. The product was ureaparticles coated with 5 wt % sodium persulfate.

These coated seed particles may be further coated with urea and with theelastomer and elastomer/thermoplastic combination as set forth above.

EXAMPLE 10

A viscous (gelled) 2 weight percent potassium chloride solution was usedto test the effects of coated breaker chemical (oxidizer) on reducingthe viscosity or breaking the gel of the viscous brine solution.

The coated breaker was compared with the base gel without breaker andwith the uncoated oxidizer added to the base gel.

The viscous brine was made by adding 50 pounds per 1000 gallons ofhydroxypropyl guar (HPG) to a 2 wt % KCl solution.

The viscosities were measured with a Fann 35 VG viscometer at 311 sec⁻¹(300 rpm) shear. Viscosities were taken during heatup to 175° F. (79.4°C.) and versus time at 175° F. Two hundred milliliters of gelled brinewas used for the tests. The coated breaker sodium persulfate at 8 and 4pounds per 1000 gallons (PPT) of gelled brine were added to the Fann 35VG meter measuring cup. Also 0.5 PPT of uncoated sodium persulfate wereadded in one test to show the effect of the SEPDM/SPS coatings on therelease of sodium persulfate with time at 175° F. The base gelled brinewith no additives were measured for comparisons.

Table III shows the advantage of coating the sodium persulfate to delaythe release of the oxidizer at a temperature of 175° F.

The uncoated sodium persulfate broke the gel to 5 cp in 30 minuteswhereas the coated sodium persulfate at 4 and 8 PPT were only 14 and 8cp after 160 minutes. Comparison with the untreated base gel shows theoxidizers all break the gel.

The film forming properties of sulfonated polymers permits them to beused in coating applications, including controlled release products inoil and gas well operations (e.g., controlled release fracturing fluidbreaker system or "breakers").

Coatings of neutralized sulfonated polymers on coated breaker chemicalswhich are used in fracturing fluids will act as barriers to watersoluble components of the fracturing fluid thereby shielding the breakerchemical from the premature release cf the breaker chemical into thefracturing fluid.

Because of their unique barrier properties sulfonated polymers can beused to make cost effective controlled release breaker chemicals. Thebenefits obtained by the use of these coatings can include laborsavings, increased breaker utilization efficiency, time savings andreduction in residual damage to the well caused by residual gels.

                                      TABLE III                                   __________________________________________________________________________    EFFECT OF COATED OXIDIZER (SODIUM PERSULFATE)                                 ON VISCOSITY OF HYDROXYPROPYL                                                 GUAR (HPG) GELLED 2% POTASSIUM CHLORIDE (KCL) SOLUTION                        __________________________________________________________________________      Viscous brine made by adding 50 pounds per 1000 gallons (PPT) of HPG to       2% KCl                                                                        solution (0.6 g HPG per 100 ml solution).                                     Viscosities measured with Fann 35 VG viscometer at 311 sec.sup.-1 (300        rpm).                                                                         Viscosities taken during heatup to 175° F. and versus time at          175° F.                                                                Two hundred milliliters of gel treated with oxidizer for                    __________________________________________________________________________      tests.                                                                      Viscosity (cp) of Gelled Solution with Oxidizers - Normalized*                                   Coated   Coated  Uncoated                                  Temperature                                                                           Time No    Oxidizer #2                                                                            Oxidizer #2                                                                           Na.sub.2 S.sub.2 O.sub.3                  (°F.)                                                                          (min)                                                                              Oxidizer                                                                            (8 ppt)  (4 ppt) (1/2 ppt)                                 __________________________________________________________________________     80          50.0  50.0     50.0    50.0                                      100          47.2  46.3     47.5    47.0                                      120          43.4  42.6     44.1    43.1                                      140          39.2  --       38.2    37.6                                      160          33.5  --       33.3    31.7                                      175      0   30.2  28.9     29.2    28.2                                               5   29.2  28.4     29.9    18.3                                              10   29.2  28.4     29.9    13.4                                              20   30.2  26.8     28.9     7.9                                              30   30.2  24.2     27.9     5.4                                              40   30.2  22.1     25.5                                                      60   30.2  18.4     22.5                                                      80   30.2  15.8     19.6                                                      100  30.2  14.7     17.6                                                      160  30.2   7.9     13.7                                              __________________________________________________________________________     Coated Oxidizer #1 (LAM 2J) Urea = 83.4%, Persulfate = 9.0%, SEPDM = 3.1%     & SPS = 2.5%                                                                  Coated Oxidizer #3 (LAM 1G) Persulfate = 82.0%, Urea = 4.3%, SEPDM = 2.0%     SPS = 2.0%                                                                    *Normalized to 50 cp at 80° F.                                    

What is claimed is:
 1. An encapsulated breaker chemical composition usedin an oil or gas stimulation operation which comprises:(a) a particulatebreaker chemical; (b) a water soluble sealing layer having a thicknessof at least about 1 micron deposited onto the surface of particles ofsaid breaker chemical and encapsulating said breaker chemical; and (c) acoating substantially free of pinholes comprising a neutralizedsulfonated elastomeric polymer having a thickness of at least about 1micron deposited onto the sealing layer surface of particles of saidbreaker chemical, wherein said neutralized sulfonated polymerencapsulates and is essentially impermeable under conditions of use tosaid breaker chemical.
 2. The composition of claim 1 wherein saidsealing layer comprises urea.
 3. The composition of claim 2 wherein saidurea layer has a thickness in the range of from about 1 to about 30microns.
 4. The encapsulated breaker chemical composition of claim 1wherein said elastomeric polymer is selected from the group consistingof sulfonated copolymers of ethylene and propylene, sulfonatedterpolymers of ethylene, propylene and a diene, sulfo Butyl rubber,sulfo isoprene/styrene rubber, sulfo isoprene/butadiene rubber, sulfoisoprene/butadiene/styrene copolymers, sulfo isobutylene/styrenecopolymers, sulfo isobutylene/para methyl styrene copolymers, andcomplexes of the aforementioned polymers with a vinyl pyridinecopolymer.
 5. The encapsulated breaker chemical composition of claim 1wherein said elastomer coating has a thickness within the range of fromabout 2 to about 50 microns.
 6. The encapsulated breaker chemicalcomposition of claim 4 wherein said neutralized elastomeric polymer is aneutralized sulfonated EPDM terpolymer.
 7. The encapsulated breakerchemical composition of claim 1 wherein the acid groups of saidelastomeric polymer are at least about 50% neutralized.
 8. Theencapsulated breaker chemical composition of claim 7 wherein the acidgroups of said elastomeric polymer are about 100% neutralized.
 9. Theencapsulated breaker chemical composition of claim 7 wherein the acidgroups of said sulfonated elastomeric polymer are neutralized with anion selected from the group consisting of zinc and sodium ions.
 10. Theencapsulated breaker chemical composition of claim 1 wherein saidbreaker chemical is coated on a seed particle which is essentiallychemically inert with respect to said breaker chemical.
 11. Theencapsulated breaker chemical composition of claim 10 wherein said seedparticle comprises urea.
 12. The encapsulated breaker chemicalcomposition of claim 1 wherein said breaker chemical is selected fromthe group consisting of ammonium persulfate, potassium persulfate,sodium persulfate, sodium chlorite and mixtures thereof.
 13. Theencapsulated breaker chemical composition of claim 1 wherein saidbreaker chemical is selected from the group consisting of sulfamic acid,oxalic acid, citric acid, ammonium sulfate, sodium acetate and mixturesthereof.
 14. The encapsulated breaker chemical composition of claim 1wherein said breaker chemical is selected from the group consisting ofammonium bifluoride, potassium fluoride, sodium fluoride and mixturesthereof.
 15. The encapsulated breaker chemical composition of claim 1wherein said breaker chemical is an enzyme.
 16. The encapsulated breakerchemical composition of claim 6 wherein said EPDM terpolymer containscopolymerized ethylidene norbornene.
 17. The encapsulated breakerchemical composition of claim 6 wherein said coating has a thickness inthe range of from about 2 to about 80 microns.
 18. The encapsulatedbreaker chemical composition of claim 1 wherein said sulfonatedelastomeric polymer contains from about 4 to about 200 milliequivalentsof pendant sulfonate groups per 100 g of polymer.
 19. The composition ofclaim 1 wherein said encapsulated breaker chemical is further overcoatedwith a layer of thermoplastic polymer.
 20. The composition of claim 19wherein said thermoplastic polymer is a neutralized sulfonated styrenepolymer.
 21. The composition of claim 20 wherein the thickness of saidlayer of thermoplastic polymer is within the range of from about 1 toabout 20 microns.
 22. A fracturing fluid comprising a carrier fluid, aviscosifying polymer or oil and an encapsulated breaker chemical asdefined in claims 1, 4, 10, or
 19. 23. A process for fracturingsubterranean formations comprising injecting under pressure a fracturingfluid into the subterranean formation, said fracturing fluid comprisinga carrier fluid, a viscosifying polymer or oil and an encapsulatedbreaker chemical as defined in claims 1, 4, 10, or 19.